The invention concerns a semiconductor power element having a safety fuse lying in its main current path according to the general class of the primary claim.
The electrical semiconductor power elements used at this time, e.g., diodes, zener diodes, or transistors, can fail at their break due to faulty craftmanship, electrical overloading, or overheating and thereby cause an electrical short circuit. In rectifier arrangements in alternators used in the automotive industry, short circuits can also occur if batteries are connected with incorrect polarity or a charging device for a motor vehicle battery is connected with incorrect polarity, which leads to an impermissibly high current flow in the main current path of the semiconductor power element. In most cases, the short-circuit current causes the semiconductor power element and surrounding components, e.g., incoming lines or coverings, to be destroyed after just a short period of time. In the extreme case, such events can also lead to overheating with consequential damages in the motor vehicle before the excessive current flow is interrupted by the destruction of the semiconductor.
Fuses are used as necessary to protect semiconductors. Common commercial safety fuses that basically can be electrically connected in series with the semiconductor power elements require special installation space and the construction expense that goes along with it. Additionally, they add electrical series resistance, which results in current-dependent heat loss.
It is already known from DE 30 01 52 2 C2 that a fuse element can be situated in a rectifier arrangement between the connections of the positive and negative power diodes of the rectifier bridges. The fuses are therefore formed by conductor segments that are bent and extend out of recesses of a printed card of the rectifier arrangement in the shape of loops that melt in half if excessive current flow occurs. Due to production tolerances, however, impermissibly great differences in the responsiveness of such fusible links occur, making them unsuitable for use, at the least, in generators having high power density.
With the present invention, the aim is to integrate a fuse as protection against electrical or thermal overload in semiconductor power elements that are as easy as possible to manufacture and that are as reliable as possible in terms of responsiveness at a specified temperature limit value.
The semiconductor power element according to the invention having the characteristic features of the primary claim has the advantage that, due to the fact that the semiconductor lead or its points of contacts are designed as a fusible link, no additional components or spacexe2x80x94and only a minimum amount of additional materialxe2x80x94is required. A further advantage lies in the fact that, when the fusible link is used, no resistance is added, nor does the additional heat loss occur in the main current path of the semiconductor power element that this would bring with it.
Advantageous further developments and embodiments result from the remaining features named in the subclaims. Since the thermal load of transistors is usually limited by controlling the amperage through the power transistor itself, it is appropriate to use the proposed solution with passive semiconductors, preferably diodes or zener diodes. They are particularly suited for use in passive semiconductors in a bridge rectifier of an alternator for motor vehicles. There, the semiconductor power elementxe2x80x94which, with its further connection opposite to the lead, is comprised of a metal housing in known fashionxe2x80x94is secured in a heat sink in current and heat-conducting fashion.
A particularly simple and reliable solution arises from the fact that the segment of the lead and/or its points of contact designed as a fusible link is embedded in a current and heat-insulating, temperature-resistant material. In reality, the rectifier power element can be equipped with a fusible link immediately after manufacture in that the contact zone of the semiconductor having the leadxe2x80x94also known as the head wirexe2x80x94is covered with a pourable sealing compound comprised of silicon. As an alternative, the upper end of the leadxe2x80x94in fact, its contact zone with a connecting lead in the main current pathxe2x80x94can also be covered with a pourable sealing compound comprised of silicon, however. This is particularly advantageous with secondary protection, e.g., in bridge rectifiers in alternators of motor vehicles. In terms of assembly, however, it is particularly advantageous if the center section of the lead lying between the two ends is enclosed in a sleeve, preferably comprised of ceramic, to form the protection. In order to adequately protect it against stresses due to shaking, it is advantageous if the sleeve is slid onto the lead in clamping fashion.
A high-quality, purposeful disconnection from excessive current flow in response to a previously determined temperature limit value is realized by a solution in which the center section of the lead is designed as a fusible link having two-stage responsiveness. The cross-section of the center section is reduced in appropriate fashion by way of plastic deformation to form a first resistor element and it is connected in parallel with a second resistor element, the melting temperature of which is lower than that of the lead. A particularly easy-to-manufacture solution provides that the center section of the lead is molded in the shape of a semicircle, and that the second resistor element is inserted in the recess created as a result. In particularly appropriate fashion, the second resistor element is thereby comprised of tin, which flows into the semicircular space in a soldering bath. An exact proportioning of the material accumulation and distribution can be achieved in advantageous fashion in that the second resistor element is a solder pellet comprised of tin and having a specified size, which is soldered into the recess of the center section. As an alternative, depending on the required responsiveness of the fusible link, the second resistor element can also be a zinc disk that is soldered into the recess of the center section using a soldering bath. A round wire made of copper is appropriately used for the lead.
In each of these cases, the specified melting temperatures of the two resistor elements ensures that, if the semiconductor power element becomes overloaded, the second resistor element comprised of tin or zinc melts in half first, which causes the current in the remaining first resistor element to increase to the extent that this resistor element then melts in half immediately as well, and the main current path is interrupted. In order to prevent the electrical resistance of the fusible link from increasing as compared to the original lead, the shape and the materials selected for use in the two resistor elements in the fusible link ensure that it has a total electrical resistance that is not greater than that of an non-deformed lead of corresponding length.